Orbital Base Station Filtering of Interference from Terrestrial-Terrestrial Communications of Devices That Use Protocols in Common with Orbital-Terrestrial Communications

ABSTRACT

An orbiting multiple access transceiver communicates with terrestrial mobile stations which are also capable of communicating with terrestrial base stations. The multiple access transceiver is configured to sample a signal when a terrestrial mobile station of interest is not transmitting to produce a sample signal. The sample signal may be processed to produce an out-of-phase signal that may be applied to a signal when the terrestrial mobile station of interest is transmitting to produce a clearer signal from the terrestrial mobile station of interest.

CROSS-REFERENCES TO PRIORITY AND RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityfrom U.S. application Ser. No. 17/167,711 filed Feb. 4, 2021, entitled“Orbital Base Station Filtering of Interference fromTerrestrial-Terrestrial Communications of Devices That Use Protocols inCommon with Orbital-Terrestrial Communications,” which claims benefit ofand priority from U.S. application Ser. No. 15/963,903 filed Apr. 26,2018, now U.S. Pat. No. 10,951,305, entitled “Orbital Base StationFiltering of Interference from Terrestrial-Terrestrial Communications ofDevices That Use Protocols in Common with Orbital-TerrestrialCommunications.”

The following applications are related:

1) U.S. Non-provisional patent application Ser. No. 15/857,073, filedDec. 28, 2017 entitled “Method and Apparatus for Handling Communicationsbetween Spacecraft Operating in an Orbital Environment and TerrestrialTelecommunications Devices That Use Terrestrial Base StationCommunications” (hereinafter “Speidel I”);

1) U.S. Non-provisional patent application Ser. No. 15/857,073, filedDec. 28, 2017 entitled “Method and Apparatus for Handling Communicationsbetween Spacecraft Operating in an Orbital Environment and TerrestrialTelecommunications Devices That Use Terrestrial Base StationCommunications” (hereinafter “Speidel I”);

2) U.S. Provisional Patent Application No. 62/465,945, filed Mar. 2,2017 entitled “Method for Low-Cost and Low-Complexity Inter-SatelliteLink Communications within a Satellite Constellation Network for NearReal-Time, Continuous, and Global Connectivity” (hereinafter “SpeidelII”); and

3) U.S. Provisional Patent Application No. 62/490,298 filed Apr. 26,2017 entitled “Method for Communications between Base Stations Operatingin an Orbital Environment and Ground-Based Telecommunications Devices”(hereinafter “Speidel III”).

The entire disclosures of applications recited above are herebyincorporated by reference, as if set forth in full in this document, forall purposes.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for thefiltering interference from terrestrial mobile devices communicatingwith terrestrial base stations that is encountered at an orbital basestation communicating with terrestrial mobile devices in general, and inparticular, filtering interference in a cellular system that usesterrestrial base stations and orbital base stations with protocols,spectrum, and/or timing that is common for the terrestrial base stationsand the orbital base stations.

BACKGROUND Overview of Mobile Communications

Mobile communication involves signals being sent between a mobilestation (MS) and a transceiver that can provide an interface for the MSto communicate to and from other network resources, such astelecommunication networks, the Internet, and the like, to carry voiceand data communications, and possibly other features such aslocation-finding. Being mobile, a portion of the communication path willbe wireless and wireless signals might combine when there are multipletransmitters using the same protocol, space, spectrum, and/or time.Because of that, the protocols used by the devices, such as mobilestations and base stations, should handle potential overlap of the useof a wireless space to accommodate multiple mobile stations as there arelikely to be multiple mobile stations. One approach is to divide anallocated spectrum into channels, divide time periods into timeslots,and divide a service region into cells, setting power levels such thatnot all devices transmit significant energy in their signals to theentire service region occupied by receivers. For example, a cellularcommunications system might operate with protocols wherein a spectrum isdivided into predefined channels, transmitters limit their transmissionsto predefined timeslots, and power levels are such that a mobile deviceuses enough power to close a link (i.e., handshake and establish a linkconnection with sufficient reliability) with a base station and not muchmore, with a geographic range of the service region divided into cellsso that a mobile device need only transmit enough to reach its nearest,or one of its nearest, base station.

A terrestrial cellular communications system comprises terrestrial basestations and mobile devices. The service region of a terrestrialcellular communications system might cover urban areas and other areaswith sufficient infrastructure and users to make a service offeringfeasible. That service region might have gaps in coverage and might notextend to all areas where users with mobile devices might need or wantcoverage. An orbital communications system comprises orbital basestations and mobile devices. The service region of an orbitalcommunications system might be able to cover more continuously somegeographic area than the terrestrial cellular communications system, oreven the entire surface of the Earth or other celestial body. Aterrestrial cellular communications system and an orbital communicationssystem might coexist in some overlapping regions by using separated andunrelated protocols and spectrum, with users of the terrestrial cellularcommunications system possibly using one type of mobile station forterrestrial cellular communications and users of the orbitalcommunications system possibly using another type of mobile station fororbital communications.

Terrestrial Cellular Communications Systems

In a terrestrial cellular communications system, a mobile device willtypically close a link with a base station that provides the strongestsignal at the mobile device's location. A base station will typicallyclose links with multiple mobile devices that are in range of the basestation. Using time and frequency division, multiple mobile devices cancommunicate with a base station at the same time. With GSM, in oneexample, the spectrum band of 890-915 MHz is used uplink communications(from mobile device to base station) and the spectrum band of 935-960MHz is used for downlink communications (from base station to mobiledevices.

Each spectrum band is partitioned into channels. Each channel has acarrier frequency and the carrier frequencies are separated by about 200kHz. In the general case, a channel might be defined by its carrierfrequency and half of its channel separation on each side, so in thisspecific case, a channel has a carrier frequency considered to becentered within the channel's 200 kHz spectrum. This provides for 125channels, and with about one channel on each side of a spectrum band asa guard separation, that leaves 123 channels for moving data. If, oneach channel, data is transmitted in frames where a frame compriseseight timeslots that can be separately allocated to mobile devices,123*8=984 mobile devices can communicate with one base station at atime. For a base station with multiple transceivers, perhaps usingdirectional antennas, one base station might be able to support somemultiple of 984 simultaneous links to mobile devices. In some instances,a base station is not ever allocated all 123 channels and in thoseinstances, the base station's capacity is less.

Using some protocol mechanism, such as the use of RACH frames, acoordinated set of base stations in a terrestrial cellularcommunications system allocates out channels and timeslots to mobiledevices that are trying to communicate with base stations. For example,a mobile device might signal its presence with a RACH request and a basestation, hearing that request, might return a reply to indicate to themobile device that it is to use timeslot t₃ (of t₀ through t₇) andfrequency f₃₇ (of f₀ through f₁₂₂).

Transceivers and Mobile Stations

The transceiver may be a component in a base transceiver station (BTS)that handles traffic from multiple transceivers. The BTS might alsoinclude antennas and encryption/decryption elements. The antennas mightbe selective antennas, wherein different MS s at different locationsmight communicate to their respective transceivers via differentantennas of the BTS. The BTS may have a wired, wireless, and/or opticalchannel to communicate with those other network resources. A BTS mightsupport one or more transceivers and a given base station for supportingmobile communication might have a base station controller (BSC) thatcontrols one or more BTS of that base station.

Examples of mobile stations include mobile phones, cellular phones,smartphones, and other devices equipped to communicate with a particularBTS. While herein the mobile stations are referred to by that name, itshould be understood that an operation, function or characteristic of amobile station might also be that of a station that is effectively orfunctionally a mobile station, but is not at present mobile. In someexamples, the mobile station might be considered instead a portablestation that can be moved from place to place but in operation isstationary, such as a laptop computer with several connected peripheralsand having a cellular connection. The mobile station might also bestationary, such as a cellular device embedded in a mounted homesecurity system. All that is required is that the mobile station be ableto, or be configured to, communicate using a mobile communicationinfrastructure.

Each of these elements could be implemented using hardware and/orsoftware and include network management and maintenance functionality,but a base station can be described as having one or more transceiversthat communicate with mobile stations according to an agreed-uponprotocol. This can be by having the BTS being configured, adapted, orprogrammed to operate according to the agreed-upon protocol for a BTSand having the MS being configured, adapted, or programmed to operateaccording to the agreed-upon protocol for a MS. The protocol mightinclude details of how to send data between a transceiver and a MS, howto handle errors, how to handle encryption, and how to send controlinstructions and status data between the BTS and the MSs. For example,parts of the protocol might include interactions wherein an MS contactsa BTS and the BTS indicates to the MS what timing, carrier frequency,and other protocol options the MS is to use. This interaction mightinclude carrying voice data, carrying text data, carrying other data,providing for intracell handover and other tasks.

Examples of BTSs include cellular telephone towers, macro-celltransceivers, femto-cell transceivers, picocells (which might have onlyone transceiver) and the like. BTSs will communicate with MSswirelessly. Some BTSs have a backhaul (the interface between the BTS andthe other network resources) that is wired, such as with a cellulartelephone tower, while some might have a wireless backhaul, such as amicrowave point-to-point bidirectional communications channel. Thus, aBTS might be any of several different types of electrically powereddevices that receives data streams from MSs and processes those and/orforwards them to other network resources, as well as receiving datastreams from the other network resources and processing those and/orforwarding them to MSs over the BTS-MS link(s). In this sense, a BTSacts as an access point for the MSs, to allow an MS to access networkresources such as a telecommunications network, the Internet, privatenetworks, etc. The access might be used to route voice calls, othercalls, texting, data transfer, video, etc.

A telecommunications network behind a BTS might include a network andswitching subsystem that determines how to route data to an appropriateBTS and how to route data received from a BTS. The telecommunicationsnetwork might also have infrastructure to handle circuit connections andpacket-based Internet connections, as well as network maintenancesupport. In any case, the BTS might be configured to use some protocolswith MSs and other protocols with the backhaul.

Mobile Communications Protocols

In many examples herein, communications is described as being between aBTS and one MS for simplicity of explanation, but it should beunderstood that the interactions might be from a BTS to a transceiver,to a radio circuit, to an antenna, to a MS antenna, to an MS radiocircuit, and to software/hardware in the MS. There is also acorresponding path in the other direction from the MS to the BTS. Thus,in some examples where a BTS is communicating with an MS, it is via atransceiver and the example ignores mention of the other transceiversthat the BTS might be controlling or otherwise communicating with.

Examples of protocols that a BTS might use include GSM (Global Systemfor Mobile Communications; trademarked by the GSM Association) 2G+protocols with Gaussian minimum-shift keying (GMSK), EDGE protocols withGMSK and 8-PSK keying. A BTS might handle multiple transceivers that usemultiple sets of carrier frequencies within a spectrum band of wirelessspectrum that the protocol allows for. Thus, where a spectrum band islogically divided into carrier frequency spectra, a transceiver mightuse channels that use one (or more) of those carrier frequencies tocommunicate with an MS. The protocol might specify that for a givenchannel, there is an uplink subchannel and a downlink subchannel, incontiguous or noncontiguous spectra. In some cases, the uplinksubchannel has a carrier frequency adjacent to that of the downlinksubchannel. In some cases, all the uplink subchannels are in onespectrum band and all the downlink subchannels are in another spectrumband. For ease of explanation, sometimes a channel is described ashaving an uplink portion and a downlink portion as if it were onechannel, even if the two subchannels of a channel are noncontiguous.

Some BTSs might provide for frequency hopping, where the transceiversand the mobile stations rapidly jump together from carrier frequency tocarrier frequency to improve overall BTS performance. The protocol mightspecify the hopping sequences to use.

In the GSM protocol, transceiver-MS communication involves frames andeach frame has up to eight timeslots. With eight timeslots, atransceiver sends out a frame that is directed at up to eight MSs, witheach MS assigned a unique timeslot in the frame by the transceiver'sBTS. The MSs can send their transmissions in their allotted timeslot andsince each MS that is communicating with that transceiver knows whichtimeslot they are to use, similarly situated MSs can communicate back tothe transceiver in their allotted timeslot. A transceiver might not useall eight timeslots.

A signaling channel, such as the GSM protocol's Common Control Channel(CCCH) might be used to convey to the MSs what their allocations are fortimeslots and channels/carrier frequencies. For example, some CommonControl Channels are used to make access requests (e.g., making RACHrequests, which are from a MS to a BTS), for paging (e.g., making PCHrequests, which are from a BTS to a MS), for access grant (e.g., anAGCH, which is from a BTS to a MS), and cell broadcast (e.g., CBCH,which is from a BTS to a MS). The AGCH (Access Grant Channel) is usedfor granting timeslot allocations/carrier allocations. Another channel,the Broadcast Control Channel (BCCH), might or might not be used to sendinformation to the MS, such as Location Area Identity (LAI), a list ofneighboring cells that should be monitored by the MS, a list offrequencies used in the cell, cell identity, power control indicator,whether DTX is permitted, and access control (i.e., emergency calls,call barring, etc.).

The protocols for communication between MSs and BTSs might be such thatthey are standardized so that any standard MS can communicate with anyBTS, assuming range requirements are met and membership requirements aremet (e.g., that the MS has identified itself to the BTS in such a mannerthat the BTS, or a service that the BTS uses, determines that the MS isa member of an authorized group or otherwise authorized to use theservices provided by the BTS. Some example protocols include the GSMprotocols, sometimes referred to as 2G (i.e., second generation) networkprotocols. Other examples include GPRS (General Packet Radio Services),EDGE (Enhanced Data rates for GSM Evolution, or EGPRS), 3G(third-generation 3G UMTS standards developed by the 3GPP body, orfourth-generation (4G) LTE Advanced protocols.

In these protocols, there are rules for spectrum band use, timing,encoding and conflict resolution. As a BTS is likely to have tocommunicate with many MSs at the same time, the available wirelesscommunication pathway is divided up according to the protocol. A givenprotocol might have the available wireless communication pathway dividedup by frequency, time, code or more than one of those. This allowsmultiple users to share the same wireless communication pathway.

For example, with a Time Division Multiple Access (TDMA), the BTS andthe multiple MSs agree on the division of time periods into timeslots(or “burst periods”) and where a first MS might interfere with a secondMS, the first MS is assigned a first timeslot and the second MS isassigned a different timeslot of the available timeslots. Sincedifferent MSs use different timeslots (and they all agree on timingsufficiently well), they can share a common carrier frequency and theirrespective transmissions do not interfere. An example would be wherethere are eight timeslots of 576.92 μs (microseconds) each for eachframe and so an MS assigned the first timeslot will perhaps transmit anumber of bits during the first timeslot, stop transmitting at or beforethe end of its timeslot, remain silent, then during the first timeslotof the next period, continue transmitting, if desired. Similarallocations occur for a MS to determine when it is to listen forsomething from a BTS (and for the BTS to determine when it is to starttransmitting that data).

Thus, using a single carrier frequency, each transceiver of a BTS cancommunicate with up to eight MSs and communications to those MSs isgrouped into a TDMA frame and transmitted on the downlink channels thatuse that carrier frequency channel. The timing is such that each ofthose MSs can communicate in their respective timeslots to the BTS onthe uplink channels that use that carrier frequency channel. This isreferred to as a “TDMA frame” and the data rate over all eight MSs usingthat carrier frequency is 270.833 kilobits/second (kbit/s), and the TDMAframe duration, in either direction, is 4.615 milliseconds (ms).

Frequency Division Multiple Access (FDMA) is another way to divide upand allocate the available wireless communication pathway. With FDMA,the spectrum bandwidth available or allocated for the wirelesscommunication pathway is divided up into different channels by carrierfrequency. A first MS might be assigned one carrier frequency and asecond MS might be assigned another carrier frequency, so that both cansend or receive to or from one BTS simultaneously.

In the above examples, a plurality of mobile stations communicate with aBTS perhaps simultaneously, wherein communication between the BTS and aspecific MS comprises sending information in a signal from the specificMS or from the BTS such that collisions of wireless signals are avoided,by having the BTS and the specific MS agree on which timeslot of aplurality of timeslots is to be used (TDMA), and/or agree on whichcarrier frequency of a plurality of carrier frequencies is to be used(FDMA). These are examples of multiple-access communications.

In another type of multiple-access communication, called “OrthogonalFrequency Division Multiple Access” (OFDMA), mobile devices are assignedsubsets of subcarriers, where orthogonal narrow frequency subchannelsare assigned to mobile devices for more efficient use of allottedspectrum compared to FDMA.

In some frequency allocations, the allocation is per channel block,where a channel block is a set, or group, of bidirectional channels,wherein each bidirectional channel uses an uplink carrier frequency foran uplink subchannel and a downlink carrier frequency for a downlinksubchannel. The channels might be grouped together into sets of two ormore channels based on some logic for classification such that each setshares a common identifier or attribute.

In some protocols, the spectrum is divided into subspectra for carrierfrequencies and also the periods are divided into timeslots. Typically,the BTS includes logic to determine which channels to allocate to whichMSs. In assigning a channel for use by a MS, the BTS might assign aparticular transceiver to use a particular carrier frequency andindicating to an MS that it is to use that particular carrier frequencyand also indicate which timeslot to use from a frametransmitted/received using that carrier frequency. The channel mightcomprise an uplink subchannel and a downlink subchannel. It may be thata given transceiver-MS communication uses more than one channel, e.g.,more than one carrier frequency and/or more than one timeslot, but inmany examples herein, the protocol is illustrated as being with respectto a MS that uses a channel comprising just one carrier frequency andjust one timeslot.

In yet another example of multiple-access communications, called “CodeDivision Multiple Access” (CDMA), mobile devices might use the sametimeslot and carrier frequency, but each mobile device is assigned aunique pseudorandom code to encode the signals to and from the BTS suchthat even when MSs simultaneously transmit using the same carrierfrequency, or almost the same time, and/or the same timeslots, if thoseare used, applying the unique CMDA code allows for multiple transmittersto occupy the same time and frequency, as the receivers can separate outdifferent receptions by decoding using the pseudorandom codes to decodeeach specific signal well enough for demodulation.

In effect, CDMA separates the channels not strictly by time or strictlyby frequency. The use of CDMA results in a transmission ofspread-spectrum signals, spread across a larger bandwidth than withoutencoding, by using a chipping rate that is faster than the signal bitrate. Thus, encoding signals with pseudorandom codes can replace thetiming and frequency elements typically found in TDMA/FDMA protocols, aseach code represents some element of articulation in both the time andfrequency domain. In CDMA communications, signal propagation delay andtiming between the MS and the BTS is understood and so the pseudo-randomcode is applied to a received signal across some number of bits/chipswhich, of course, occupy both some discretized span of the time domainand some discretized span of the frequency domain.

In some multiple-access protocols, more than one approach is used.

In GSM protocol digital mobile radiotelephone systems, MSs and BTSsleverage communications across both frequency and time division multipleaccess (FDMA/TDMA) channels such that MSs can share the same transmitand receive carriers via the assignment of distinct timeslots over eachcarrier frequency and each carrier frequency might be handled by adistinct transceiver or transceiver module or logic block.

In GSM, the BTS is responsible for assigning a timeslot to the mobilestation (MS) when it requests access. In a GSM frame structure, thereare eight timeslots within each TDMA frame. The number of carrierfrequencies used can vary. In some regions, some carriers are licensedfor a large number of carrier frequencies and MSs in those regions areconfigured to accept instructions to use one of as many as a thousandcarrier frequencies (which a BTS would also support). For instance, inEurope the GSM 900 MHz spectrum band comprises 25 MHz of spectrum. Ifthis is logically allocated into 200 kHz channels (e.g., each channelhaving a carrier frequency centered within the channel and the carrierfrequencies each separated by 200 kHz so that a channel is associatedwith a 200 kHz subspectra band), and transceivers send signals on thosechannels, this provides for 125 channels. The use of guard bands (unusedcarrier frequencies) in the frequency domain might reduce this number,but might provide added reliability or ease of signal processing. Wherea TDMA frame allows for eight timeslots, a BTS having sufficient numbersof logical or actual transceivers available, could support 8*125=1000MSs channels simultaneously. With time division and frequency division,there can be guard slots and guard frequencies, respectively, so thatone division has some separation from an adjacent division. With someprotocols, more than one timeslot and/or more than one carrier frequencycan be assigned to one MS, to provide greater bandwidth.

In some cases, there are multiple BTSs within range of supported MSs andso the support of the MSs can be spread among the BTSs and perhaps theycoordinate so that adjacent BTSs avoid using the same carrierfrequencies when possible. BTSs might be programmed to spread thesefrequencies across their towers with a specific reuse scheme. It mightalso be that a BTS is limited in the number of MSs it can support by thesize of the pipe to the other network resources. In one example, a BTSuses from 1 to 15 carrier frequencies (i.e., its transceivers transmitusing 1 to 15 carrier frequencies in sending/receiving frames, so itcould support anywhere from 8 to 120 simultaneous users).

Each MS typically includes a processor, memory, radio circuitry, a powersource, display, input elements and the like to perform its functions.The processor might read from program memory to perform desiredfunctions. For example, the program memory might have instructions forhow to form a data stream, how to pass that to the radio circuitry, howto read an internal clock to determine the value of a system clock toappropriately time listening and sending, and how to set appropriatefrequencies for transmissions and reception.

Each BTS typically includes a processor, memory, radio circuitry, powersource(s), interfaces to the telecommunications network, diagnosticinterfaces and the like to perform its functions. The BTS processormight read from program memory to perform desired functions. Forexample, the program memory might have instructions for how to form adata stream, how to pass that to the radio circuitry, how to communicatewith the telecommunications network, how to read an internal clock todetermine the value of a system clock to appropriately time listeningand sending, how to set appropriate frequencies for transmissions andreception, how to keep track of the various MSs and their state,location, allocation, etc. and perhaps store that into locally availablememory.

In the manner described above, an MS will contact a BTS to get allocatedsome timeslots in frames in some carrier frequencies and the BTS willinform the MS of the MS's allocation. As both the BTS and the MS havethe same system clock (or approximately so), they will communicatewithin their allotted timeslots and carrier frequencies. The assignmentand communication of the assignments to the MSs might occur using arandom access channel that is used by the MS to request an allocation.In the GSM protocol, this is referred to as a RACH process.

In the example of GSM, communication over the wireless communicationpathway is parsed into TDMA frames of duration 4.61538 ms, with eighttimeslots per TDMA frame. Each timeslot is long enough to hold 156.25bits of data. In one application, the MS or BTS will transmit 148 bitsof data in a timeslot, over 546.46 μs, with 8.25 bits (30.46 μs) of aguard time between timeslots. In the GSM900 Band, the wirelesscommunication pathway has a bandwidth of 25 MHz in the uplink anddownlink directions each, using the spectrum band of 890-915 MHz foruplink subchannels and the spectrum band of 935-960 MHz for downlinksubchannels, providing for 125 carrier frequencies (125 carrierfrequencies in each direction, spaced 200 kHz apart). With 200 kHz ofguard separation on each side of each spectrum band, that leaves 24.6MHz of spectrum, or 123 carrier frequencies, for moving data. The totalcapacity of such a wireless communication pathway (in both directions)would then be 156.25 bits per timeslot times eight timeslots per frametimes 216.667 frames/second*123 carriers=33.312 Mbits/second.

Once a mobile device is assigned a channel and a timeslot, it can sendbursts of data to the base station, perhaps by encoding around 156.25bits of data by modulating its channel's carrier frequency signal usingGaussian Minimum Shift Key (GMSK) modulation. With GMSK, the modulatedcarrier frequency signal spreads out over 200 kHz, dropping in amplitudefurther away from the carrier frequency and having somewhat of a null200 kHz away from the carrier frequency on either side.

The arrangement of terrestrial base stations are typically such that twomobile devices that are assigned the same channel and the same timeslotare separated enough that their communications with their respectivebase stations is separated by enough distance that the signal energyfrom one mobile device is greatly attenuated around the other mobiledevice's base station. The arrangement of “cells” allows for this. Thismight also provide separation for adjacent channels, so that mobiledevices using the same timeslot and adjacent channels are separatedgeographically so that even as their energy is spread into the adjacentchannel, it is attenuated.

It might be that not every base station allocates links on everychannel. Since coverage areas of base stations would overlap to ensurecontinuous connections, adjacent base stations might coordinate so thatthey do not use the same channels. For example, if base stations arearranged geographically in a hexagonal pattern and the 123 channels aregrouped into channel groups A, B, and C of 41 channels each, one basestation could limit itself to assigning channels in channel group A,while its six closest neighbor base stations alternate among channelgroups B, C, B, C, B, C. In that case, two adjacent base stationswouldn't use the same channels. In other arrangements, there are 7 or 9channel groups and their reuse is coordinated among base stations. Inthe latter case, there is considerable separation between two basestations using the same channel group and so the signals from the basestation and mobile devices having links to that base station are wellseparated from the other base station and the mobile devices it issupporting.

In the above approach, transmissions from mobile stations can bemaintained with separation by timeslot, frequency, a geographicalseparation for terrestrial cellular communications, but if the sameprotocols are used for orbital mobile communications, it might be thatthe orbital base stations would have to contend with large numbers ofsignals from mobile stations that are communicating with nearbyterrestrial cellular base stations, and due to an orbital base station'slarger footprint, the orbital base station's transceiver picks up manymore transmissions than a typical terrestrial cellular base station.

SUMMARY

An orbital base station (OBS) using protocols in common with terrestrialcellular base stations (TCBSs) receives signals from mobile stations,both those that are using links, or establishing links, with the orbitalbase station and those that are using links, or establishing links, witha terrestrial cellular base station. The orbital base station iscommunicating with, or initiating communications with, a target mobilestation. The orbital base station processes a first received signalduring a first period, a sounding period, and derives a sounding basesignal. During the sounding period, the target mobile station is notsending a communication. During a second period, a signaling period, theorbital base station processes a second received signal at leastincluding subtracting a representation of the sounding base signal fromthe second received signal to derive a filtered signal that is thenprocessed to determine data being communicated by the target mobilestation. This can be done even if there are many more mobile stationscommunicating with their respective TCBS while in a footprint of theorbital base station.

In yet another approach, the first received signal during the soundingperiod is distilled into a sounding base signal that collectivelyrepresents signals from a plurality of mobile base stations that arecommunicating with TCBSs using channels adjacent to, or within, a targetchannel that the orbital base station has assigned to the target mobilestation. The sounding base signal is similarly subtracted from thesecond received signal to cancel out, or mostly cancel out, thosesignals destined for the in-footprint TCBSs to make it easier to decodethe communications from the target mobile station.

In some variations, the sounding period and the signaling period areboth timeslots in a frame and in other variations, the sounding periodis a timeslot in one frame and the signaling period is a timeslot in anadjacent frame. In some variations, the sounding period is not alwaysthe same timeslot. For example, the sounding period might be timeslot 0of frame N and the signaling periods are timeslots 1-7 of frame N, onesounding period might be timeslot 0 of frame N for the signaling periodof timeslot 0 of frame N+1, another sounding period might be timeslot Mof frame N for the signaling period of timeslot M of frame N+1, thesounding periods might be timeslot 0 of frame N, then timeslot 6 offrame N+4 with the other timeslots being signaling periods, or someother variation. In some cases, the statistics over the in-footprintmobile stations communicating with TCBSs might be such thatcollectively, the output from timeslot to timeslot is similar enoughthat different variations of sounding periods and signaling periodsmight be sufficient for use.

In various examples, the sounding period is illustrated as preceding thesignaling period, but the signaling period could come before thesounding period, although this might delay processing of the desiredsignal until after the sounding period is complete.

In some implementations, a central communication management system mightmanage both TCBSs and orbital base stations and coordinate so that whenthe central communication management system determines that an orbitalbase station would have a particular footprint, it might direct anorbital base station to use a first set of frequencies and/or a firstset of timeslots and direct the TCBSs that are in that footprint to usea second set of frequencies and/or a second set of timeslots, so as toreduce interference. The central communication management system mightalso manage frequencies such that there are unused frequencies betweenthe first set of frequencies and the second set of frequencies.

The central communication management system might be implemented as partof the orbital base station, in the ground environment, or both. Thespace and ground segments of the networks might each have their own corenetwork, or central communication management system, that connects,communicates, or cooperates with each other to accomplish thesefunctions.

Other methods of generating the sounding base signal could be usedinstead of, or in addition to, those above in order to generate asounding base signal that represents, or models, a profile of theinterference/RF that mobile stations communicating with their TCBSswould impose on signals received at an orbital base station from mobilestations that are communicating with the orbital base station using aprotocol that is in common with the mobile stations communicating withtheir TCBSs.

It is also possible that the interference signal energy being profiledis not entirely of a standard protocol. It might comprise signalsgenerated using different types of protocols, as in the instance of across-border satellite footprint, wherein one protocol and one portionof the spectrum might be used on the one side of a border while oneprotocol and one portion of the spectrum might be used on the one sideof a border while another protocol and another portion of the spectrumis used on the one side of a border and both portions are received atthe orbital base station. Each side may be different, perhaps as aresult of different licensing/allocation of the spectrum for differentuses. It could also be because the spectrum is allocated to differentmobile network operators that use the frequencies for differentcommunications technologies (e.g., LTE, CDMA, GSM, etc.) wheremodulation schemes, bit rates, channel bandwidths, etc. are not alwaysthe same.

In a specific embodiment, a difference between the sounding period andthe signaling period is that in the sounding period there are no signalsreceived at the orbital base station that correspond to communicationsthat the orbital base station needs to process, but include signalsreceived from mobile stations communicating with their TCBSs on thesurface, among other interference, whereas during the signaling period,there are signals received at the orbital base station that correspondto communications that the orbital base station needs to process alongwith a similar interference profile described above that might beexhibited in the sounding period.

The sounding process might be repeated over additional sounding periods,in part to account for changing conditions. The sounding process (andalso the signaling process) might be done by obtaining an RF signal,converting it to a baseband relative to one or more carrier frequency toobtain a baseband analog signal, digitizing the baseband analog signal,and using digital signal processing techniques to manipulate thedigitized signal to generating the sounding base signal. For example,Fourier transforms, such as via a Fast Fourier Transform (FFT) method,might be used to convert from the time domain to the frequency domain,and corresponding inverse Fourier transforms, such as via an InverseFast Fourier Transform (IFFT) method, might be used to convert from thefrequency domain to the time domain. Various appropriate sampling ratesmight be used in digitizing the baseband analog signal.

However obtained, the sounding base signal is generated, it can beprocessed before using it in combination with the second received signalreceived during the signaling period. For example, it might be processedin the frequency domain to attenuate frequencies far from the basebandfrequency of the second received signal. The combination of the soundingbase signal and the second received signal can be by subtracting thesounding base signal from the second received signal or by inverting thesounding base signal and then adding it to the second received signal.The sounding signal might be sampled more than once and recorded as aseparate vector of amplitudes and phases that represent the waveforms inthat bandwidth sample.

In these manners, the effective SINR of the second received signal canbe improved, facilitating an orbital base station in communicating withmobile stations that are transmitting to the orbital base station usingthe same or similar protocol, frequency ranges, timeslots, etc. asmobile stations that are within a footprint of the orbital base stationbut are communicating with TCBSs and have some of their transmittedenergy impinging on a receiver of the orbital base station.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments in accordance with the present disclosure will bedescribed with reference to the drawings, in which:

FIG. 1 is an illustration of an orbital base station over a terrestrialsurface having TCBSs and mobile stations within a footprint of theorbital base station with some of the mobile stations communicating witha TCBS and other mobile stations communicating with the orbital basestation, each using a common protocol and/or common range offrequencies, timeslots and the like.

FIG. 2 is an illustration of frequency division of a spectrum intochannels.

FIG. 3 is a plot of signal power of a GMSK modulated RF signal as afunction of frequency around its channel's carrier frequency.

FIG. 4 is a plot of signal power of a GMSK modulated RF signal as afunction of frequency around a channel's carrier frequency for oneGSM/GPRS signal on one channel and for two adjacent GMSK modulated RFsignals on adjacent GSM/GPRS channels.

FIG. 5 illustrates the GSM/GPRS frame structure, which is a TDMA/FDMAprotocol that might be used by a mobile station for communicating eitherwith a TCBS or an orbital base station.

FIG. 6 illustrates how RF traffic might be shared acrosschannels/carrier frequencies and timeslots in a telecommunicationsnetwork; FIG. 6A illustrates uplink transmissions from mobile stationsto a base station, while FIG. 6B illustrates downlink transmissions froma base station to mobile stations.

FIG. 7 illustrates a terrestrial cellular base station network and howchannels on that network might be reused between adjacent base stationswithin the network.

FIG. 8 illustrates how the coverage area, or footprint, of an orbitalbase station may overlap regions of operation of TCBSs.

FIG. 9 illustrates how an orbital base station might perceive signals onits uplink carrier frequency channels relative to adjacent carrierfrequency channels when the orbital base station has a dedicated carrierfrequency channels and mobile stations using TCBSs use the adjacentcarrier frequency channels.

FIG. 10 illustrates the signal energy levels that an orbital basestation might receive for the orbital base station's channel andadjacent neighboring channels, taking into account signals from amultiplicity of mobile stations that are communicating with the TCBSs,such as those TCBSs shown in FIG. 8 .

FIG. 11 illustrates a significant amount of RF signal energy in adjacentchannels introducing significant spurious interference in a channel ofinterest such that a signal of interest in this channel is challengingto demodulate.

FIG. 12 is a block diagram of portions of a satellite that handlescommunications.

FIG. 13 is a schematic diagram of the DSP section of FIG. 12 in greaterdetail.

FIG. 14 illustrates where a sounding period is in one timeslot andsignaling periods are in other timeslots, with a TDMA frame structure.

FIG. 15 illustrates how an RF signal is sampled and digitized to berepresented in the digital domain as bit level values.

FIG. 16 illustrates the results of processing steps that might occur inthe digital signal processing illustrated in FIG. 13 .

FIG. 17 illustrates, in the frequency domain, how the digital profilenulls out interference from coherent signal energy that originates inthe wanted channel or adjacent channel.

DETAILED DESCRIPTION

In the following description, various embodiments will be described. Forpurposes of explanation, specific configurations and details are setforth in order to provide a thorough understanding of the embodiments.However, it will also be apparent to one skilled in the art that theembodiments may be practiced without the specific details. Furthermore,well-known features may be omitted or simplified in order not to obscurethe embodiment being described.

Base stations in a wireless communications system can support multiplemobile stations in that they can correctly decode communications frommultiple mobile stations and can transmit signals that respective mobilestations can correctly decode, even though there are many mobilestations overlapping and some base stations that overlap with other basestations. One or more of frequency division or time division can beused, as well as cellular division or carrier frequency re-use, wherepositions of various base stations and power levels used are such thatcommunications between one mobile station and one base station do notinterfere with communications between another mobile station and anotherbase station.

An orbital base station might be one function of a satellite. Thesatellite, since it operates across such a wide coverage area, it mightalso have infrastructure on orbit corresponding to core infrastructureof a cellular network, such as in the case of a GSM/GPRS network,operating as a “network-in-a-box” where it employs BTS, BSC, MSC(perhaps G-MSC), VLR, HLR, EIC, AuC, etc. functionalities.

Where terrestrial base stations and orbital base stations are both usedand might be in range of some common set of mobile stations, and themobile stations use the same protocol or similar protocols tocommunicate with terrestrial base stations and orbital base stations,problems can arise in receiving signals at an orbital base station.There could also be terrestrial devices that do not use the sameprotocol but operate within the satellite footprint, perhaps as a resultof difference in spectrum/frequency allocation across borders and/or useof spectrum/frequency in an unlicensed/unregulated/illegal manner.

Terrestrial base stations and the mobile stations using thoseterrestrial base stations can use lower power relative to orbital basestations due to the differences in distances (such as 35 km forterrestrial and 500 km for orbital) so the coverage area of oneterrestrial base station does not overlap that many other coverageareas. Orbital base stations also have much larger footprints becausethey have much wider geographical areas of view as their line of sightto a horizon is much longer than for a terrestrial tower.

Additionally, with an orbital base station, higher power is required andso the coverage area within which mobile stations transmissions areheard by the OBS is much greater. This could be easily solved byrequiring mobile station-to-OBS communications to use different hardwarethan terrestrial communications, different spectra, or the like.However, that can be undesirable when wanting to implement a cellularcommunications network wherein mobile devices can connect to an OBS inthe same manner as a terrestrial base station. As used herein, “TCBS”refers to a terrestrial cellular base station, which is a base stationthat has limited coverage and other base stations take up for othercoverage area, thus creating “cells” of coverage. In examples describedherein, ORB s operate as base stations in orbit and appear to a cellularcommunications network's mobile stations (such as mobile devices,smartphones, etc.) as just a normal terrestrial cellular base station.This type of cellular infrastructure, or base station in orbit, isdescribed in Speidel I.

Transmissions from mobile stations can be maintained with separation bytimeslot, frequency, a geographical separation for terrestrial cellularcommunications, but if the same protocols are used for orbital mobilecommunications, it might be that the orbital base stations would have tocontend with large numbers of signals from mobile stations that arecommunicating with nearby terrestrial cellular base stations, and due toan orbital base station's larger footprint, the orbital base station'stransceiver picks up many more transmissions than a typical terrestrialcellular base station.

Transmissions from base stations are not as much of a problem forinterference. Base stations listen on uplink channels and transmit ondownlink channels. An orbital base station does not normally need tolisten on downlink channels, so transmission from terrestrial basestations would not interfere with operation of an OBS. Also,transmissions to mobile stations are not normally a problem where bothterrestrial cellular base stations and orbital base stations are in use.A mobile station will close a link with a closest base station andcloseness relates to signal strength. If a mobile station is in an areawith no terrestrial coverage, it will connect with an OBS and therewould be little interference as other nearby mobile stations would alsolikely have no coverage and would connect with the OBS. If a mobilestation is in an area with good terrestrial coverage, it will connectwith a terrestrial base station that is closest and will ignore arelatively weaker OBS signal. In highly populated areas withinfrastructure, a large majority of mobile stations may fall withinrange of coverage of a terrestrial base station and a small minority ofmobile stations in lesser populated areas might require an orbital basestation connection.

A difficulty can arise when an OBS is listening to uplink channels andencounters many (maybe thousands) of mobile devices that arecommunicating with terrestrial base stations yet the OBS receives someof their signal energy. With one or two such mobile devices, those cansafely be ignored, but with thousands in an adjacent channel, that mightspill over sufficient signal energy into a desired channel to raise theinterference so that the signal-to-noise-plus-interference ratio (SINR)is too low to allow a receiver from demodulating a signal of interestwithin the channel of interest at a desired bit error rate.

The base station in orbit will be subject to RF signals from a largenumber of devices on the ground that are within its field of view, orfootprint, and are operating across a swath of, potentially contiguous,carrier frequencies. Under this operating condition, the orbital basestation might use a certain uplink carrier frequency that is adjacent toone, or more than one, uplink carrier frequency being used by basestation infrastructure on the ground. In this case, the satellitereceiver will be challenged to demodulate uplink signals on its carrierfrequency of interest due to the excess interference. This is becausecarrier frequencies are reused abundantly among terrestrial basestations within the satellite footprint so adjacent carrier frequenciesmay comprise a significant number of RF signals from mobile devices onthe ground. Spurious energy from adjacent carriers may createsignificant enough interference in the uplink carrier of interest toprevent signal demodulation.

There could potentially be a very large number of mobile stations withinthe OBS's footprint, however, that are radiating RF on uplink channelsbeing used to communicate with the terrestrial base stations in thecommunications network. Some of these mobile stations could be radiatingRF in a channel that is adjacent to the uplink channel on which theorbital base station is expecting to receive signals. Since the largemajority of mobile stations on the ground will be within coverage ofterrestrial base stations, the number of signal bursts within thosechannels will significantly outnumber the number of signal bursts in theorbital base station channel of interest. When the orbital base stationuplink channel, or set of channels, is adjacent to channel used by theterrestrial base stations, the signals within the channel of interestwill be subject to a lot of spurious interference. This spuriousinterference will degrade the SINR on the OBS uplink channel and makesignal demodulation challenging, or impossible.

Furthermore, there could be signal energy from carriers that areoperating on the orbital base station uplink channel as well. In otherwords, there could be signal energy from carriers within the uplinkchannel of interest in addition to energy from adjacent channels. Thetechniques described herein can reduce interference. One method is toremove coherent energy in the channel of interest. In the examplesherein, it might be assumed that there are multiple channels and, ineach channel, communication occurs by modulating a carrier wave that hasan unmodulated frequency that is centered, more or less, in the channel.Energy is then dispersed over the channel based on the modulation usedand might extend beyond the nominal boundaries of the channel.Typically, for one signal on one channel, the bulk of the energy iswithin the channel boundaries so as to not overwhelm signals in adjacentchannels.

In various examples herein, mobile devices are communicating with an OBSand mobile devices communicating with TCBSs such that the OBS sees amuch wider population of mobile devices, in part because the OBS isoperating from orbit and there is greater distance between the mobiledevices and the OBS and TCBSs need only support a much smallerfootprint. The examples herein that refer to OBSs might also be used insimilar arrangements wherein a base station has a much widerpopulation/footprint than the TCBSs but where the base station is notnecessarily in orbit. It might be used in other situations where a widerpopulation/footprint needs to be supported, such as supporting a hugestadium full of people, using an airplane as a base station, using alarge tower as a base station and other scenarios. Thus, it should beunderstood that the examples herein might be extended as appropriate.

FIG. 1 is an illustration of an orbital base station over a terrestrialsurface having TCBSs and mobile stations within a footprint of theorbital base station with some of the mobile stations communicating witha TCBS and other mobile stations communicating with the orbital basestation, each using a common protocol and/or common range offrequencies, timeslots and the like. As illustrated there, terrestrialmobile stations, such as mobile devices 102, are in use on Earth surface104. Some of these mobile devices, such as mobile devices 102(2), 102(4)and 102(5), are communicating with terrestrial cellular base stations106, but also send signal energy 110 in the directions of orbital basestations such as orbital base stations 108(1) and 108(2). This caninterfere with communications between other mobile stations and anorbital base station, such as mobile device 102(1) and orbital basestation 108(1) and mobile device 102(3) and orbital base station 108(2).The mobile devices can have, or be, programmable transceivers. The OBScan have, or be, a programmable transceiver, as can the TCBSs.

GSM as an Example Protocol

The embodiments below are in an order in which the process may beimplemented to better explain the details. One embodiment uses theGSM/GPRS protocol and a Fast Fourier Transform for interference signalcharacterization. Other embodiments may use other communicationsprotocols such as LTE, EDGE, CDMA, etc. Other embodiments may use signalprocessing algorithms other than a Fast Fourier Transform.

GSM/GPRS is a time division multiple access (TDMA) and frequencydivision multiple access (FDMA) protocol. In GSM/GPRS, a plurality ofmobile stations communicates with a base station, perhapssimultaneously, wherein communication between the base station and aspecific mobile station comprises sending information in a signal fromthe specific mobile station or from the base station to avoid collisionsof wireless signals. The base station and the specific mobile stationagree on which timeslot of a plurality of timeslots will be used (TDMA)and which carrier frequency of a plurality of carrier frequencies willbe used (FDMA). This is an example of a time division multiple access(TDMA) and frequency division multiple access (FDMA) protocol.

FIG. 2 is an illustration of frequency division of a spectrum intochannels. As illustrated there, a predetermined, or agreed-upon,spectrum is divided into channels. In this example, there are 125channels. Each channel is considered to be 200 kHz wide. While thesignal in a channel is greatest within the bounds of the channel, it isnot necessarily zero outside of the channel bounds.

In the GSM protocol, transceiver-mobile station communication involvesframes that have up to eight timeslots. With eight timeslots, atransceiver sends out a frame that is directed at up to eight mobilestations, with each mobile station assigned a unique timeslot in theframe by the transceiver's base station. The mobile stations can sendtheir transmissions in their allotted timeslot. Because the transceiverassigns each mobile station that is communicating a timeslot, similarlysituated mobile stations can communicate back to the transceiver intheir allotted timeslot. A transceiver might not use all eighttimeslots.

In the GSM protocol, there are rules for spectrum band use, timing,encoding and conflict resolution. As a base station is likely to have tocommunicate with many mobile stations at the same time, the availablewireless communication pathway is divided according to the protocol.

In GSM, for example, with Time Division Multiple Access (TDMA), the basestation and the multiple mobile stations agree on the division of timeperiods into timeslots (or “burst periods”). To avoid interferencebetween a first mobile station and a second mobile station, the firstmobile station may be assigned one timeslot and the second mobilestation may be assigned a different timeslot of the available timeslots.Since different mobile stations use different timeslots (and they allagree on timing sufficiently well), they can share a common carrierfrequency and their respective transmissions do not interfere. As anexample, if there are eight timeslots of 576.92 μs (microseconds) eachfor each frame, a mobile station assigned the first timeslot maytransmit a number of bits during the first timeslot, stop transmittingat or before the end of its timeslot, remain silent, then during thefirst timeslot of the next period, continue transmitting, if desired.Similar allocations occur for a mobile station to determine when tolisten for something from a base station (and for the base station todetermine when it is to start transmitting that data).

Thus, using a single carrier frequency, each transceiver of a basestation can communicate with up to eight mobile stations. Communicationto those mobile stations is grouped into a TDMA frame and transmitted onthe downlink channels using that carrier frequency channel. The timingis such that each of those mobile stations can communicate in theirrespective timeslots with the base station on the uplink channels thatuse that carrier frequency channel. This is referred to as a “TDMAframe”. The data rate over all eight mobile stations using that carrierfrequency is 270.833 kilobits/second (kbit/s), and the TDMA frameduration, in either direction, is 4.615 milliseconds (ms). Each TDMAframe, therefore, consists of 1,250 bits where each TDMA timeslot cancarry up to 156.25 bits.

Description of the GSM/GPRS Frequency Structure

Frequency Division Multiple Access (FDMA) is another way to divide andallocate the available wireless communication pathway. The FDMA protocoldivides the spectrum bandwidth available for wireless communication intodifferent channels by carrier frequency. For example, a base stationmight assign a first mobile station one carrier frequency and assign asecond mobile station another carrier frequency, so that both can sendor receive to or from the base station simultaneously. In GSM/GPRS eachcarrier frequency occupies 200 kHz of bandwidth.

Globally, GSM/GPRS networks are allocated 4 bands: GSM850 Band, GSM900Band, GSM1800 MHz Band, and GSM1900 Band. Each band is allocated somebandwidth for uplink and downlink carrier frequencies. For instance, inthe GSM900 Band, the wireless communication pathway has a bandwidth of25 MHz in the uplink and downlink directions each, using the spectrumband of 890-915 MHz for uplink portions and the spectrum band of 935-960MHz for downlink portions, providing for 125 carrier frequencies (125carrier frequencies in each direction, spaced 200 kHz apart). With 200kHz of guard separation on each side of each spectrum band, that leaves24.6 MHz of spectrum, or 123 carrier frequencies, for moving data. Thetotal capacity of such a wireless communication pathway (in bothdirections) would then be 156.25 bits per timeslot times eight timeslotsper frame times 216.667 frames/second×123 carriers=33.312 Mbits/second.

Description of Network Frequency Reuse Schemes

When deploying terrestrial communications networks, uplink and downlinkcarrier frequencies may be allocated to each base station such as tominimize interference. Each base station is deployed to provide coverageto a geographic area that is usually known. To ensure that mobilestations can transition or hand off from one base station to another,adjacent base stations are positioned close enough such that theircoverage areas overlap slightly. This allows mobile stations to receivebroadcast signals from multiple base stations when operating near anedge in the network. This helps avoid a mobile station disconnectingwhen performing a handoff or otherwise transitioning from base stationto base station in the network.

Description of Carrier Signal Energy and Adjacent Channel Interaction

In a GSM/GPRS network, mobile devices and base stations articulate RFbursts using GMSK modulation. GMSK modulation, or Gaussian Medium ShiftKeying modulation, is a continuous phase frequency shift keyingmodulation scheme in which the phase is changed between symbols and aconstant signal amplitude envelope is maintained (reducing powerconstraints on transmitting mobile stations and base stations). A GMSKmodulated signal is created, in simple terms, by putting an MSK (MediumShift Keying) signal through a Gaussian filter.

The GMSK modulation used in the GSM/GPRS protocol has a 0.3bandwidth-time product, which defines how the signal power profile, indB, falls off, or decreases, as a function of the ratio betweenfrequency offset from the carrier and the signal bit rate,f_(off)/R_(b), where f_(off) is the frequency offset from the carrierand R_(b) is the bit rate. As the bandwidth-time product decreases, thesignal energy falls off more quickly. This creates a narrower signalenergy profile as a function of frequency and that helps to mitigateinterference between adjacent carrier frequencies. Specifically,BT=f_(−3dB)/R_(b), where BT is the bandwidth-time product, f_(−3dB) isthe frequency offset from the carrier frequency that has a signal powerlevel −3 dB down from the carrier frequency, and R_(b) is the signal bitrate at f_(−3dB).

The bandwidth-time product of a GMSK signal is also related to how thesignal pulse is spread over time during transmission. As thebandwidth-time product decreases, the time over which a signal is spreadduring transmission increases. A longer transmit time for each pulse cancreate interference between consecutive symbols that are transmittedwithin the same burst. As a result, the bandwidth-time product used inthe communication chain carries a sensitive balance that trades spectralinterference challenges for symbol interference challenges, and viceversa. In the case of GSM/GPRS, the bandwidth-time product is 0.3 sosymbols are pulsed over approximately 3 bit periods centered about thebit that the pulse seeks to represent.

FIG. 3 illustrates a GMSK modulated GSM/GPRS signal 302 in the frequencydomain as a function of frequency around its channel's carrierfrequency. GSM/GPRS uses GMSK modulation with a bandwidth-time productequal to 0.3. With a bit rate of 270.833 kbps, the signal drops in powerby half or more (−3 dB) at frequencies that are outside of the range ofthe carrier frequency+/−the −3db frequency (f_(−3dB)), which in thiscase is 81.25 kHz. In FIG. 3 , the signal energy level is definedrelative to the peak of the carrier signal. In a typical signal in a GSMchannel, the peak of the signal energy is at the channel's carrierfrequency and drops off to around −9 dB at 100 kHz away from the carrierfrequency on either side, and around −41 dB at 200 kHz away from thecarrier frequency on either side. This latter distance, 200 kHz, is thedistance to the next channel's carrier frequency, so adjacent channelinterference is around −41 dB and can often be easily handled. This isillustrated in FIG. 3 , with one channel labeled “Channel C” and theadjacent channels labeled “Channel C−1” and “Channel C+1”.

FIG. 4 is a plot of signal power of a GMSK modulated RF signal as afunction of frequency around a channel's carrier frequency for oneGSM/GPRS signal on one channel and for two adjacent GMSK modulated RFsignals on adjacent GSM/GPRS channels. FIG. 4 illustrates how adjacentcarrier frequency signals, when received at the same time, may avoidinterference with each other. When sent according to the protocolspecified in the GSM/GPRS specification, interference from adjacentchannels is low. In FIG. 4 , a channel 402 carries the bulk of theenergy of a modulated signal centered around a carrier frequency with afrequency distribution 404 relative to energy from adjacent channels,which have modulated signals centered around their carrier frequencieswith frequency distributions 406, 408. The spurious energy from theadjacent channel's signals is reduced by nearly −40 dB at the carrierfrequency. The GSM/GPRS specification requires the carrier tointerference ratio to be 9 dB in order to pass clear voice traffic.

FIG. 5 illustrates how the GSM/GPRS protocol uses a frame-basedstructure to handle communications between an orbital base station 506and mobile station 504 over ground-to-orbit link 508 and a terrestrialbase station 510 and mobile station 504 over ground link 512. A TDMAframe is 4.61538 ms in duration and contains eight timeslots. Eachtimeslot is 576.92 microseconds in duration and a modulated signal sentduring one timeslot might encode for up to 156.25 bits.

A GSM/GPRS network is designed to handle RF traffic simultaneouslyacross contiguous carrier frequencies, spaced 200 kHz apart, withdivisions by timeslot within each carrier frequency. The movement ofthis traffic is illustrated in FIG. 6 , with FIG. 6A illustrating uplinktraffic from mobile devices 606 to a base station 604 and FIG. 6Billustrating downlink traffic from a base station 608 to mobile devices610. As shown, one dimension is carrier frequency and the otherdimension is time. Each carrier frequency, such as carrier frequency 612can accommodate eight mobile devices, each using one of the eighttimeslots. In each timeslot, there is an RF signal burst, such as RFsignal burst 602. FIG. 6 depicts how the signal energy of adjacentcarrier RF signals might be perceived by a hypothetical terrestrial basestation.

FIG. 7 illustrates the allocation of uplink and downlinkchannels/carrier frequencies to base stations in a GSM network, or otherterrestrial cellular communications system, base stations provideservice to mobile stations and each base station has a region (a spaceor area, depending on how it is considered) but is typically illustratedas a shape of an area on a surface over which the base station providessupport. These might be circles, hexagons, or other shapes and might becircularly asymmetric due to terrain, interfering buildings or objects,and/or directionality of antennas. To provide continuous support formobile stations and smooth handoff, the regions will usually overlapamong the base stations. For the purposes of examples and illustrationsherein, they are shown herein as nonoverlapping hexagons, but it shouldbe understood that there may be overlap and they might not be sosymmetric. Where base stations are evenly spaced and mobile devices areassigned to base stations strictly on a closest-distance basis, then thecoverage areas might all be hexagons, but that is not a requirement. Ingeneral, the regions can often be described as having a radius ofcoverage more or less centered on a base station.

A base station might be part of a cell phone tower housing and poweringbase station transceiver equipment. In some cases, a base station mightsupport distinct region sectors, perhaps somewhat overlapping, butperhaps pointed in different directions, via the use of directionalantennas. Since mobile stations may move, the network design allows basestations to hand off connections of mobile stations as they move fromregion to region and thus their support moves from base station to basestation. To ensure seamless handovers, base stations may be placedwithin close enough proximity of their neighboring base stations suchthat their coverage zones overlap slightly. This way, in the case of ahandover, a mobile station can receive signals from more than one basestation at once so that it knows to which base station it is beinghanded over.

Since neighboring base station coverage areas may overlap, the networkcan be designed to minimize interference between adjacent base stationsand the mobile devices communicating with them. To accomplish this,neighboring base stations might refrain from using adjacent and/orcommon carrier frequencies over adjacent cells. Frequencies areallocated so that common frequency carriers, in both the uplink anddownlink direction, are repeated far enough away from each other thatmutual interference is minimized, with overlap minimized or avoidedaltogether. Channel reuse configurations might be dependent on coveragecharacteristics. In some configurations, channels/carrier frequenciesare reused every 3, 7, or 9 base stations.

This is illustrated in FIG. 7 . Cell tower base stations 708 provideservice for defined coverage areas (three of which are labeled 702, 704,706). The frequency reuse scheme defines how many base stations in anetwork reuse the same uplink and downlink carrier frequencies. Afrequency reuse scheme of 3, for instance (as is shown in FIG. 7 ),reuses the uplink and downlink carrier frequencies every third basestation in the honeycomb configuration, such that no base station withthe same uplink and downlink carrier frequencies are directly adjacentto each other. Base stations with solid white coverage areas (e.g.,coverage area 702), diagonal hatched coverage areas (e.g., coverage area704), and horizontal hatched coverage areas (e.g., coverage areas 706)are each allocated a unique set of carrier frequencies (e.g., f₁, f₂,f₃; f₄, f₅, f₆; and f₇, f₈, f₉, respectively). Here, f_(i) mightrepresent the i^(th) channel using the i^(th) carrier frequency or thei^(th) set of carrier frequencies (e.g., a channel comprises multiplecarrier frequencies). The number of carrier frequencies that might beallocated to one base station, as an example, could be anywhere from 1to 15. Frequency reuse schemes of 7 and 9 are also common.

Description of the an Example Orbital Base Station

The GSM/GPRS specification reduces interference between adjacentterrestrial stations, but does not help a base station receiving a largeplurality of signals from a large plurality of mobile stations in one ormore adjacent carrier frequencies of one or more base station uplinkcarrier frequencies. For example, a GSM/GPRS base station operating inan orbital environment would have a coverage footprint that spans acrossa significant geographic area that might contain a significant number ofterrestrial base stations.

FIG. 8 illustrates an example of the coverage an orbital base stationmight provide as a base station to a terrestrial communication network.Terrestrial towers, such as tower 808, would provide coverage in cells.As described above, the towers may be allocated different carrierfrequencies, or carrier frequency sets, for their coverage areas, asindicated, such as coverage areas 802, 804, and 806. The orbital basestation, which provides a wide radius of coverage 810, may be allocatedits own unique carrier frequency or set of frequencies. From thedownlink perspective, the orbital base station signals are originatingfrom so far away that their energy level at the surface would be similarto the energy level perceived at the edge of a terrestrial base stationcounterpart. The result is that the satellite footprint providescoverage in areas where the terrestrial towers do not and does notinterfere with existing terrestrial coverage. The carrier frequenciesallocated to the orbital base station may be perceived as weaker thanavailable terrestrial tower signals.

The uplink direction of communications may present interference issues.FIG. 9 illustrates how an orbital base station might perceive traffic onthe various carrier frequencies used by the network. Consider an orbitalbase station, 906, communicating mobile station 908 solely within itsfield of coverage (and not in coverage of a terrestrial base station)using a carrier frequency denoted f₂. Also within that field of coverageis a network of terrestrial base stations, 904, communicating with aplurality (perhaps thousands, or more) of mobile stations, 902, using aset of carrier frequencies denoted by f₁, f₃, f₄, . . . , f_(n). In thisexample, each carrier frequency is denoted in order such that f₁ and f₃are the lower and upper adjacent carrier frequency neighbors to f₂.Since f₂, 916, is allocated to the orbital base station it is only beingused by mobile stations communicating with that base station. In thecase of this uplink carrier, each time frame is allocated to, at most,eight mobile stations on the ground which would fill eight timeslotswithin the frame, 912. However, f₁, f₃, f₄, . . . , f_(n) are allocatedfor use by a multiple terrestrial base stations communicating withmultiple mobile stations on the ground that are within the orbital basestation coverage footprint. Because of this, the orbital base stationreceives the aggregated signal energy of all RF bursts articulated onthose carrier frequencies. For instance, f₃, 914, from the perspectiveof the orbital base station, comprises many, perhaps thousands, of RFsignal bursts that overlap each other across the entire time frame, 910.As a result, the aggregated signal energy level of adjacent carrierfrequencies, f₁ and f₃, could be significantly higher than the energylevel of the signals from mobile stations communicating with the orbitalbase station using the carrier frequency of interest, f₂. In some cases,even non-adjacent channels can cause interference and that can be dealtwith similarly.

Description of the Adjacent Carrier Frequency Interference

Coherent signal energy that creates interference may originate fromanywhere in the frequency domain. Coherent signal energy from adjacentcarriers may create interference on a particular carrier of interest,and coherent signal energy from other devices within the carrier ofinterest itself may also create interference.

The OBS takes advantage of the fact that the interference from theradiating energy below is relatively consistent in amplitude and phaseover time, and nearly constant over very small amounts of time (such asthe time of a GSM TDMA frame, 4.61538 ms). Although the satellite ismoving very quickly in orbit, its position relative to signal sources isrelatively consistent over short time frames. Furthermore, the majorityof signal energy generating the interference environment will be comingfrom what can effectively be considered point sources in space. Mosttelecommunications traffic will radiate from population dense areas, ormetropolises. Considering the example of New York, N.Y., USA, it coversabout 780 square kilometers, which is about the area of a circle with aradius of 15 km. From an orbit of, say, 500 km, this 15 km radius circleon the ground would correspond to about one degree of angular offsetfrom nadir. Furthermore, signals coming from each edge of the city wouldonly need to propagate an additional 0.07 km (one-way) before hittingthe spacecraft. This means that a bit that is radiated from the edge ofthe city will arrive about 233 nanoseconds after a bit that is radiatedfrom the center of the city. This difference in time is about 6% of abit period (which is 1/270.833 kbps=3.69 microseconds). Thus, the errantsignals (i.e., signals picked up at the orbital base station that arebetween mobile devices on the ground and base stations on the ground)from that circle can be treated as all originating from a single point.With multiple such urban centers, each being treated as distinct singlepoints, this may form an interference pattern of sorts, but as explainedherein, that interference pattern would be expected to be not varyingmuch over short timeframes.

Since there are a multitude of signals any given bit radiated from thecity will be a 1 or a 0, represented by some phase of the carrierfrequency at that bit. So the aggregate of RF signals coming from onecity is effectively the same as two signals being radiated continuouslyon the same frequency but offset by a constant phase difference. As aresult, each population center will generate interference that iscoherent and since the distance between the satellite and eachpopulation center below it is about the same over very short periods oftime, the amplitude of the signals generating this interferenceenvironment are relatively consistent.

The magnitude of this issue related to adjacent carrier frequencyinterference can be estimated with a set of assumptions andcalculations. In one embodiment, the orbiting base station is assumed tooperate in a circular orbit at 500 km. The coverage footprint of theorbital base station may be a near circular or other conic geographicsection of the earth's surface. A typical coverage area for acommunications satellite is related to the minimum elevation angle atwhich the satellite in orbit can create a sufficient connection with amobile station on the ground. Devices located at a slant range fartherthan this elevation angle might be discounted as their signals wouldlikely be significantly attenuated by antenna pointing offset losses.

The minimum elevation angle is defined as the angle above the horizonthat the satellite must be in order for the mobile station tocommunicate with the orbital base station. For example, a minimumelevation angle of 90 degrees (where the satellite is substantiallyoverhead and the mobile station is at the surface point in the directionof the nadir of the satellite) produces a coverage area of approximatelya single point on the Earth's surface. When the elevation angle is lowerthan 90 degrees, the coverage area expands radially, or approximatelyso. For some minimum elevation angle at which a mobile station maycommunicate with the orbital base station, the angle will generallycorrespond to the longest supported distance for such communications.

The Earth central angle, λ, of a coverage area of an orbital basestation is as shown in Equation 1, where R_(e) is the radius of theEarth, ε_(min), is the minimum elevation angle, and h is the satellitealtitude.

$\begin{matrix}{\lambda = {{a{\cos\left( \frac{R_{e}{\cos\left( \varepsilon_{\min} \right)}}{R_{e} + h} \right)}} - \varepsilon_{\min}}} & \left( {{Eqn}.1} \right)\end{matrix}$

For R_(e)=6370 km, ε_(min)=40 degrees, and h=500 km, the Earth centralangle is around 4.74 degrees. The Earth central angle is the angle thatdefines the width of the conic section, or spherical cap, of the Earth'ssurface that the coverage footprint of the satellite is defined by. Inother words, the Earth central angle is the radius of the satellitefootprint, in degrees, relative to the center of the earth. The actualsquare kilometers of surface area of satellite coverage that is definedby this Earth central angle can be closely approximated by Equation 2.

A _(f)=2πR _(e) ²(1−cos(λ))  (Eqn. 2)

When R_(e)=6370 km and λ=4.74 degrees, A_(f) is around 872,700 km² forthe coverage area of the footprint. The radius of this spherical cap maybe estimated from the Earth central angle measured in radians times theradius of the Earth, or around 527 km. Meanwhile, the coverage radius ofa terrestrial base station may be anywhere between 1 km and 35 km. Inthis example, assume a terrestrial base station with a coverage radiusof 5 km. The coverage area of a terrestrial base station is also,technically, a small spherical cap on the Earth's surface. However,since the radius of coverage capability for a terrestrial cell tower issignificantly smaller in magnitude compared to the radius of the Earth,the terrestrial base station coverage area can be approximated as a flatdisk. Therefore, the coverage radius of a terrestrial base station canbe closely approximated by the area of a circle of radius 5 km, oraround 78.5 km².

The ratio of the satellite coverage area and the coverage area of atypical terrestrial base station provides a reasonable approximation ofthe number of terrestrial cells that would fit within one satellitefootprint at one time; 872,700 km²/78.5 km²=11,118 terrestrial basestations per satellite footprint. This formula assumes that thesatellite footprint area is completely saturated with terrestrial basestations. From an operational perspective, this might be an unusual casebecause one operating principle of the orbital base station might be tofill in the gaps of the terrestrial base station coverage areas.However, the orbital base station should be capable of providingcoverage even in small gaps on the terrestrial base station coverage.This means that the orbital base station may, for some embodiments, bedesigned to sufficiently handle interference when its coverage footprintis even 99.9% saturated with terrestrial base station coverage areas.

In this example, the terrestrial base stations may use a carrierfrequency reuse scheme of 3, which is the same as illustrated in FIG. 8. Also in this example, two of the carrier frequencies used in thenetwork on the ground are adjacent to the carrier frequency used by theorbital base station. This is the same as the embodiment illustrated inFIG. 9 in which the orbital base station uplink carrier is flanked onboth the upper and lower sides by uplink carriers that are used byterrestrial base stations. If each terrestrial base station is operatingat full capacity (i.e., all timeslots within each TDMA frame are beingused), we can approximate the number of RF bursts within each timeslotof each adjacent carrier as perceived by the orbital base station. Sinceevery 3^(rd) terrestrial base station within the satellite footprintwill use the adjacent uplink carrier from the perspective of thesatellite, we can compute the number of simultaneous signal bursts bydividing the number of terrestrial base stations within the satellitefootprint by 3: 11,118/3=3,706 simultaneous RF bursts per adjacentcarrier. It should be noted that the orbital base station will actuallybe subject to more simultaneous RF bursts than 3,706 per adjacentcarrier. The reason for this is because the satellite will have a viewangle of the earth's surface well beyond the 527 km coverage radiuspreviously calculated based on the minimum elevation angle. However, thecommunication capability of the orbital base station and mobile stationson the ground is limited to the 527 km radius of coverage because of,for example, the directivity of the antenna in use (on the satellite)and degradation of signals originating from low elevation angles.Signals originating outside of this 527 km radius footprint maytherefore significantly attenuate whether or not they are interferencesignals or signals of interest. Therefore, in the following example,those signals are ignored in the interference analysis.

FIG. 10 illustrates the signal energy levels that an orbital basestation might receive for the orbital base station's channel (Channel Cin the figure) and adjacent neighboring channels (Channels C−1 and C+1in the figure), taking into account signals from a multiplicity ofmobile stations that are communicating with the TCBSs, such as thoseTCBSs shown in FIG. 8 , and adjacent carrier interference from theperspective of the orbital base station. The orbital base station uplinkcarrier frequency signal energy 1002, is 10*log₁₀(3,706), or 35.7 dBlower than the signal energy from its adjacent carrier frequencies 1004and 1006 because 1004 and 1006 may each comprise 3,706 simultaneoussignal bursts while 1002 comprises, in this example, one single burst.Accounting for two adjacent channels, the energy is10*log₁₀(2*3,706)=38.7 dB, so the SINR is around 40 dB-38.7 dB, or 1.3db. The result is that the adjacent carrier frequencies carry enoughenergy to reduce the SINR ratio on the uplink carrier frequency to 1.3dB.

Note that in a large urban area, approximately 3700 mobile stationscommunicating with their TCBSs can appear to an orbital base station asbeing in the same place and having the same propagation delay, or asmall enough variation that it can be treated that way. This would meanthat during a sounding period when communications to the orbital basestation from mobile stations is on hold, the timing of the errantsignals from those 3700 mobile stations is going to appear synchronized.Being synchronized, if they are all using the same protocol, withtimeslots divided up into bit transmission periods, such as shown inFIG. 5 , statistically half might be expected to be transmitting a zeroand the other half transmitting a one. While the individual bits mightchange from bit sample time to bit sample time (in the example of FIG. 5, the bit sample time would be around 0.577 ms/156.25=3.7 μs), theerrant ground signals picked up by the orbital base station may bestatistically consistent from timeslot to timeslot and from frame toframe. This has its limits, of course, as the orbital base station aftersome minutes have passed would be encountering a different terrestrialsituation.

The similarity between the sounding signal that is recorded during asounding period (when there are no mobile stations trying to transmit tothe orbital base station) that is caused by collective errant signalsfrom large numbers of mobile stations communicating with their TCBSs onthe ground and the collective errant signals that would occur during asampling period (when there are mobile stations transmitting to theorbital base station) commingled with the desired transmissions to theorbital base station need not be exact. The better the match, the bettera filtering out that can be performed, but it might be sufficient thatthe sounding signal in the sounding period be somewhat close to theerrant signals during a sampling period, as it can be sufficient thatthe errant signals be reduced to the point where the orbital basestation can still extract the desired transmissions.

In some embodiments, the transmissions to the orbital base stations aremade to be distinct from the terrestrial base station communications, orare necessarily so. For example, at some distances at some times, thepropagation delays of terrestrial errant signals result in them beingdifferent enough from the signals between mobile stations and theorbital base station that the errant signals in the sampled signal inthe sampling period are distinct from the signals between mobilestations and the orbital base station. However, this might not berequired.

FIG. 11 illustrates how the problem of adjacent carrier interferencecould affect a GSM/GPRS base station's performance in orbit. The carrierfrequencies, 1108 and 1110, for Channels C−1 and C+1, respectively,which are adjacent to the carrier frequency of interest, 1106 (thecarrier for Channel C), are being used by a significant number of mobilestations in the terrestrial base station network. The result is thatthere is a high amplitude signal level, 1102, in the adjacent carrierfrequencies, which creates interference within the carrier frequency ofinterest, 1104. As FIG. 11 suggests, the signal of interest may onlybarely exceed, if at all, the interference level at the carrierfrequency of the channel of interest. In these scenarios, the SINR,1112, is so low that demodulating the signal at a desirable bit errorrate becomes challenging or impossible. In some applications, this isinsufficient margin for GMSK signal demodulation at, for example, a GSMbit rate of 270.833 kbps with a reasonable bit error rate of, forexample, less than 1%.

Digital Signal Processing

FIG. 12 is a block diagram of portions of a satellite that handlescommunications. As shown there, a satellite 1200 houses an orbital basestation. An antenna 1202 receives signals from within a footprint asdescribed elsewhere herein. Antenna 1202 provides an analog signal to anRF analog receiver 1204. The carrier frequency of interest is suppliedto the RF analog receiver 1204. Note that there might be more than oneof such receivers, one per channel, implemented in hardware and/orsoftware. An output of the RF analog receiver 1204 is a baseband analogsignal, which is supplied to an analog-to-digital converter (A/D) 1206.The digital signal is processed by a digital signal processor (DSP) 1208that outputs a bitstream from a mobile station to a processor 1210 thatcan then process the binary code from the bitstream perhaps in a PHYnetwork layer 1212 in a conventional manner.

In this manner, a multiple access transceiver, adapted for operation inEarth orbit and configured for communication with terrestrial mobiledevices that are also capable of communicating with terrestrial basestations, receives a signal from some of terrestrial mobile devices, afiltering module reduces a portion of the signal due to a plurality ofterrestrial mobile devices that are communicating with terrestrial basestations to produce a filtered signal comprising a signal from aparticular mobile device communicating with the multiple accesstransceiver, and a signal demodulator demodulates the filtered signal toproduce a demodulated signal corresponding to a signal from theparticular mobile device.

FIG. 13 is a schematic diagram of the DSP section of FIG. 12 in greaterdetail. As illustrated there, a sounding signal, as might be receivedfrom A/D 1206 (shown in FIG. 12 ) during a sounding period, is providedto a Fourier transform block 1302, which might be implemented inhardware or software. The output of Fourier transform block 1302 isprovided to a lowpass filter (LPF) block 1304 that might attenuateportions of the signal that are too high frequency, which wouldcorrespond to portions of the signal that are in channels that are farfrom the channel of interest. An example might be a low pass filter of600 kHz, which may be implemented digitally using a software definedradio or otherwise. Other frequency domain processing can be done atblock 1306, such as effecting a complement by adjusting by the Fouriertransform of negative one. The output of block 1306 is provided to IFTblock 1308, which converts back to the time domain to provide for asounding base signal. This can be stored for later use in storage 1310.

A later use is in a signal period, when a sampled signal (such as beingsampled by A/D 1206 of FIG. 12 ) in the signal period is applied to anadder 1312 that adds the stored sounding base signal to cancel out theeffects from unwanted signals to form a filtered signal. The filteredsignal is then provided to a demodulator 1314 that outputs phase values(constellation points) to a GMSK demapper 1316 that outputs a bitstreamthat might form the output of DPS 1208 shown in FIG. 12 .

In the present embodiment, a digital signal processing (DSP) step in thereceiver chain mitigates the interference from adjacent carrier signalenergy and increases the signal of interest SINR in the digitalenvironment.

Description of Channel Sounding and Profiling RF Interference

The GSM/GPRS protocol uses 200 kHz wide carrier bandwidths that aresegmented into TDMA frames, each comprising eight timeslots, orchannels, per TDMA frame. In the example below, channel sounding (orsampling) is performed on the first timeslot within a TDMA frame,although channel sounding could be implemented on any timeslot ortimeslots. The sounded timeslot is left unassigned to any mobilestations communicating with the receiver so that the receiver is onlyreceiving interference signals and noise during that slot. This soundingprocedure is done so that the receiver can generate a digital profile ofthe interference environment and use that profile to create an“out-of-phase” counterpart for the interference environment. This“out-of-phase” counterpart is then used to process the remainingtimeslots in the TDMA frame to reduce the effect of adjacent carrierfrequency interference and increase the SINR within the carrierfrequency of the orbital base station. The process for how this“out-of-phase” counterpart for the interference environment may be usedfor multiple types of algorithms that can characterize a digitalwaveform. The amplitude measurement might be used to generate a basesignal for interference reduction, in addition to the phase or insteadof the phase.

The channel sounding, or sampling, process may be a sample of thedesired carrier bandwidth and its adjacent carrier neighbors, possiblyrepeated more than once. In this embodiment, the first timeslot couldpossibly be long enough for multiple soundings, or samplings. Multiplemeasurements of the interference energy might better inform theout-of-phase counterpart created to mitigate interference. The moreinformed the model is, the better the interference mitigation can be.

FIG. 14 illustrates a GSM/GPRS TDMA frame and its format. In the exampleof FIG. 14 , the channel sounding procedure is performed during thefirst timeslot 1402. The remaining timeslots 1404, 1406, 1408, 1410,1412, 1414, and 1416 may be assigned to mobile stations for uplink andmight contain signals to be demodulated, 1418, 1420, 1422, 1424, 1426,1428, and 1430. While the first timeslot 1402 is illustrated as beingempty, terrestrial communications might still be happening in thattimeslot.

The channel sounding performed in timeslot 1402 provides the receiverwith a profile of the interference from adjacent carrier frequencies.This profile is used, via digital signal processing, to generate adigital out-of-phase counterpart for the interference profile. By addingthis digital out-of-phase counterpart to the signals received in theremaining timeslots, 1404, 1406, 1408, 1410, 1412, and 1414, theinterference from adjacent carrier frequencies is significantly reducedand the signals of interest are more accurately demodulated.

In one embodiment, a snapshot bandwidth spectrum sample of the wantedcarrier of 200 kHz and the two adjacent carriers (approximately 600 kHzwide) can provide enough channel waveform data to generate a processedmodel that can cancel the unwanted RF in the carrier frequency ofinterest. Modern signal processing technology may perform near real-timeor real-time sampling and processing of 600 kHz using relatively littlepower, so this particular embodiment is not power intensive.

In some embodiments, the sounding period(s) and signaling periods have adifferent relationship, such as where the sounding period that is usedto take a sounding that is used to reduce the terrestrial interferenceduring a signaling period has the same timeslot as that signalingperiod, but from a different frame, such as the preceding frame. Inother variations, the sounding periods might vary timeslots and mightnot be needed for every frame.

Process for Generating Out-of-Phase Counterpart to InterferenceEnvironment

The process of generating the out-of-phase counterpart to theinterference profile may be completed using a host of differentalgorithms to characterize a digital signal waveform. This particularembodiment uses the Fourier transform to characterize the soundedinterference signal waveforms, but other algorithms may be used in placeof or in addition to the Fast Fourier Transform.

FIG. 15 illustrates sampling an RF signal during the channel soundingoperation on the first timeslot of the GSM/GPRS TDMA frame. The analogsignal, 1502, is received by the receiver (shown in FIG. 15 as abaseband signal) and sampled at a rate of t_(s) ⁻¹, where t_(s) is thetime between samples (i.e., the sample time). The analog signal issampled at this rate over a time period, denoted as T_(s), equal to onetimeslot period, or 577 microseconds. The result of this sampling isstored in the computer as a discretized vector of values of the signallevel, 1504, which is of length N=T_(s)/t_(s). Each of the N values inthe vector comprises some number of bits that correspond to some signalenergy level in dB or amplitude. This vector is the digitalrepresentation of the received signal in the time domain. The processdescribed above can be done with a receiver front end, ananalog-to-digital signal converter, and a computer with memory to storethe sampled, or measured, values.

Once the signal has been converted to the digital environment, thereceiver is left with a vector of values that represent a signal energylevel, or amplitude, over some span of time equal to 577 microseconds induration. This vector of signal energy values, denoted as kW, is adiscretized representation of the received signal as a function of time.The computer in the orbital base station will put this signal, {rightarrow over (s)}(t), through a digital signal processing block togenerate the out-of-phase counterpart of the interference profile,denoted {right arrow over (s)}(t). This digital signal processing blockuses the Fourier transform to fingerprint {right arrow over (s)}(t) inthe frequency domain, generate a corresponding out-of-phase fingerprint,and then use the inverse of the Fourier transform to generate {rightarrow over (s)}(t).

The Fourier transform that is used to generate a frequency domainrepresentation of the sampled signal can be implemented in software orhardware. The input to the Fourier transform is a digitizedrepresentation of the sounding signal in the time domain sampled at asampling rate, such as a value for each sampling time (regularly spacedin time at the multiplicative inverse of the sampling rate) over thesampling period, where each value represents the input signal at thatcorresponding sampling time (such as an amplitude or energy level),perhaps as determined by an A/D. The output is a digitizedrepresentation of the signal in the frequency domain, with a complexpair for each represented frequency that might be stored as an amplitudeand a phase of the signal at that represented frequency with the spacingbetween represented frequencies determined from the sampling rate in aconventional manner.

In the example above, when an entire timeslot is used for the soundingperiod, the sampling period would be T_(s)=577 μs. With the input signal(energy or amplitude) being sampled with an assumption that signalenergy more than 300 kHz away on either side of the carrier frequency(600 kHz total) is not relevant, or has been filtered out, a sufficientsampling rate is 1.2 million samples/second (twice the bandwidth of 600kHz), giving a suitable sampling time of t_(s)=0.833 μs. In that case,the total number of samples would be T_(s)/t_(s)=692.4 input signal timevalues. For simplicity, 700 input time samples might be used.

The 700 input time samples can be easily stored in memory as a timedomain (TD) representation of the sounding sample. Using a Fouriertransform process on those 700 input time samples, 700 complex values inthe frequency domain might result, which also can be easily stored inmemory for later use. Those 700 complex values represent magnitude andphase at frequencies spaced over the 600 kHz bandwidth (from −300 kHz to300 kHz). A discrete Fourier transform operation might be performed,where the 700 input time samples are real values (or complex values withthe imaginary parts assumed to all be zero) and the output is the 700complex values. There might also be a process for interpolating to morethan 700 complex values, such as 100 complex values, or reducing thenumber of complex values. However done, this results in a frequencydomain (FD) representation of the sounding sample derived from the timedomain (TD) representation of the sounding sample. That FDrepresentation can be stored in memory. The TD representation and the FDrepresentation can be stored in the same memory or different memories,perhaps accessible by a processor that can perform operations referencedhere.

In the above example, the ratio of sampling to filter is three to one,in that the signal is sampled at 600 kHz, which is a channel and its twoadjacent channels, and filtered to one channel width (200 kHz). In othervariations, the ration is other than three, such as sounding over fivechannels, seven channels, or some even multiple of 200 kHz or even anon-multiple of 200 kHz. Also, in some embodiments, the sounding periodneed not be exactly one timeslot long, but might be different parts of atimeslot or less or more than 577 μs worth of samples.

Phase Base Sounding Signal Measurement

A processor might perform a phase base sounding signal measurement asfollows. From the FD representation, the processor can bandpass filterthe FD representation by attenuating the complex values corresponding tofrequencies outside a range of −100 kHz to 100 kHz to form a filtered FDrepresentation. This bandpass filtering might be done by simply zeroingthe two-thirds of the values of the FD representation outside of thebandpass range (possibly also adjusting the phase values of theremaining samples accordingly).

Using an inverse Fourier Transform with the filtered FD representationrepresenting the 200 kHz of bandwidth of interest, the processor cangenerate a reconstructed TD representation that might by 700 real-valuedenergy or amplitude values, back in the time domain. The reconstructedTD representation can be stored as a sounding base signal for the signalbandwidth of interest.

The sounding base signal can be subtracted from a time domainrepresentation of a signal captured from a signal period using vectoraddition. In this manner, the sounding base signal can be filtered outof the captured signal and used for cancelling interference that isknown to be coherent, or of relatively unchanging phase over smallperiods of time.

Amplitude Base Sounding Signal Measurement

Alternatively, another type of sounding base signal could be assumed tobe the amplitude portion of the filtered FD representation complexvalues representing the 200 kHz of bandwidth of interest. Theseamplitude portions might be stored as real values in memory, and thensubtracted from the amplitude portions of a frequency-domain filteredrepresentation of a signal captured during a signal period. TheAmplitude base sounding signal might be useful in cancellinginterference that is known to have relatively constant signal amplitudeover small periods of time.

Phase and Amplitude Base Sounding Signal Measurement

There could also be some combination of a phase based and amplitudebased sounding signal that is used to generate a cancellation effect forthe interference environment in the 200 kHz bandwidth of interest. Amethod like this using both amplitude and phase of the sounding signalmight be useful in cancelling interference that is known to be ofrelatively constant phase and amplitude over short periods of time.

Example Process Flow

FIG. 16 provides a flow summary of the process of processing steps thatmight occur in the digital signal processing illustrated in FIG. 13 ,using a phase base sounding signal for canceling the interference inother timeslots. The process flow, 1602, indicates that the horizontalaxis represents time, specifically, timeslot number, denoted by 1604,1606, 1608, 1610, 1612, 1614, 1616, and 1618. The vertical axisrepresents process steps conducted within each of those timeslots. 1620represents the analog signal that the receiver receives. That analogsignal is processed through an analog to digital converter, 1622. Theresult of that process is 1624 and in the case of the first timeslot,the signal is processed through a digital signal processing (DSP) block.This DSP block calculates the out-of-phase counterpart, 1630, as it isdescribed in the language above. The signals from timeslots 1606-1718are added digitally, 1628, to the out-of-phase counterpart, 1630. Theresult of this operation is a clearer signal of interest, represented by1632. A signal demodulator 1634 is used to demodulate the processedsignal 1632 to generate a demodulated signal comprising perhaps a datapacket of 1s and 0s, 1636.

FIG. 17 illustrates how the embodiment mitigates adjacent carrierinterference in the frequency domain. When signals of interest arrive,the receiver takes in an analog signal which includes interference andthe signal of interest, 1702. The energy from the interference signals,1704, is digitally removed using the out-of-phase interferencecounterpart from the interference sounding process. The result of thisprocess might yield something like 1706 where the original signal ofinterest, 1708, has a much higher SINR.

1.-12. (canceled)
 13. A method of processing signals received by asignal processor of an orbital base station received from a plurality ofterrestrial mobile devices, the terrestrial mobile devices also capableof communicating with terrestrial base stations, the method comprising:a transceiver capable of receiving a signal from some of the terrestrialmobile devices; a filtering module that reduces a portion of the signaldue to a transmitting plurality of the terrestrial mobile devices thatare communicating with one or more of the terrestrial base stations,wherein the filtering module produces a filtered signal comprising asignal from a target mobile device, which is distinct from thetransmitting plurality, communicating with the orbital base station; anda signal demodulator that demodulates the filtered signal to produce ademodulated signal corresponding to a signal from the target mobiledevice.
 14. The method of claim 13, wherein the signal processor of theorbital base station is housed within the orbital base station in Earthorbit.
 15. The method of claim 13, wherein the demodulated signal isfurther processed to produce a binary code.
 16. The method of claim 13,further comprising: determining a sounding period, during which thetarget mobile device is not sending a communication to be received bythe signal processor; receiving a sounding signal during the soundingperiod; digitizing the sounding signal to form a digitized soundingsignal; processing the digitized sounding signal with a Fouriertransform to form a transformed sounding signal; and filtering thetransformed sounding signal to form a filtered sounding signal whereinportions of the filtered sounding signal are attenuated based onfrequency separation relative to a baseline frequency.
 17. The method ofclaim 13, wherein processing the filtered signal further comprisesapplying an inverse Fourier transform to the filtered signal to form atransformed sounding signal, the method further comprising: storing thetransformed sounding signal for later use; in a signal period, applyingthe transformed sounding signal and a sampled signal to an adder,thereby canceling out effects from unwanted signals, to form thefiltered signal; providing the filtered signal to a demodulator thatoutputs phase values as constellation points; and processing theconstellation points using a demapper to generate an output bitstream.18. The method of claim 13, further comprising: segmenting the filteredsignal into TDMA frames, each comprising a plurality of timeslots perTDMA frame, with at least one of the TDMA frames designated a soundingtimeslot that is left unassigned to any of the plurality of terrestrialmobile devices and other timeslots assigned as signaling timeslots;generating a profile of an interference environment based on soundingsignals received in the sounding timeslot; creating an out-of-phasecounterpart for the interference environment; and processing thesignaling timeslots in the TDMA frame using the out-of-phasecounterpart.
 19. The method of claim 18, wherein the sounding timeslotand the signaling timeslots are the same timeslot relative to TDMAframing from different frames.
 20. The method of claim 18, furthercomprising processing the sounding signals to determine a phase basesounding signal measurement as follows, determined by bandpass filteringa Fourier transform domain representation of the sounding signals. 21.The method of claim 20, wherein bandpass filtering the Fourier transformdomain representation comprises attenuating complex values of theFourier transform domain representation outside a designated frequencyrange to form a filtered FD representation.
 22. The method of claim 13,wherein the filtering module samples a frame in which the target mobiledevice is known to not be transmitting.
 23. The method of claim 13,wherein the target mobile device does not transmit during a first timeperiod during which the transceiver sounds a channel to produce a samplefrom the signal.
 24. The method of claim 23, wherein the filteringmodule converts the sample to a discretized vector in a time domain. 25.The method of claim 23, wherein the filtering module processes thesample using a Fourier transform to produce an out-of-phase counterpartof the sample.
 26. The method of claim 23, wherein the filtering moduleprocesses the sample to produce an out-of-phase counterpart of thesample.
 27. The method of claim 26, wherein the transceiver receives aportion of the signal during a second time period that does not overlapthe first time period.
 28. The method of claim 27, wherein the filteringmodule applies the out-of-phase counterpart to the portion to producethe filtered signal.
 29. The method of claim 28, wherein theout-of-phase counterpart to is applied to the portion by summing theout-of-phase counterpart with the portion.
 30. The method of claim 23,wherein the filtering module processes the sample to measure amplitudeto generate a base signal for interference reduction.
 31. The method ofclaim 23, wherein the filtering module processes the sample to measureamplitude and phase to generate a base signal for interferencereduction.